1. Field of the Disclosure
This disclosure relates to an apparatus and a method for estimating a condition of formation fluid using an index of refraction during withdrawal of such fluid from the formation.
2. Description of the Related Art
Oil and gas wells are drilled while circulating drilling fluid (also referred to as “mud”) in the wellbore. Drilling fluids are typically water-based or oil-based. After drilling the well and before completing the well for the production of hydrocarbons, fluid samples are often withdrawn from the subsurface formation at various wellbore depths to determine the characteristics of the fluid in order to determine the location and fraction or amount of hydrocarbons in the formation fluid and the condition of the reservoirs, etc. In some cases, it also is desirable to obtain samples from the formations or zones that contain mostly formation brine, i.e., water samples.
A majority of the wells are drilled under overbalanced conditions, that is, the wells are drilled wherein the pressure in the wellbore due to the weight of the drilling fluid is greater than the formation pressure. The drilling fluid invades or penetrates to varying depths in the formation, depending upon the physical conditions of the formation being drilled, such as porosity, permeability and other rock properties. This fluid penetration (also referred to as the fluid invasion) contaminates the connate or virgin fluid in the formation. Therefore, before obtaining a fluid sample downhole, a tool is set at the desired depth and the fluid is withdrawn or pumped from the formation into the wellbore until it is determined that the fluid being withdrawn is substantially free of the drilling fluid. Downhole tools referred to as “formation testers” are typically set at the desired depth in the wellbore to pump out the fluid and to withdraw the formation fluid samples. Initially, fluids that are withdrawn from the formation are often highly contaminated with filtrates of the drilling fluid used for drilling the wellbore. To obtain samples that are sufficiently clean (usually <10% contamination) formation fluids are generally pumped from the formation into the wellbore for a period of time, typically 30-90 minutes, before collecting samples in sample chambers for laboratory analysis. Optical sensors are often used to monitor a contamination level in the withdrawn fluid. Optical absorption measurements have been used to estimate how long it might take before relatively clean fluid samples can be taken and to estimate the eventual purity or contamination levels if the fluid is pumped for a relatively long time period. Refractive index measurements have been taken downhole but have not been used to estimate purity or contamination levels for brine. Refractive index measurements can be much less sensitive to the passage of sand particles or other elements present in the formation fluid that may scatter light in the fluid being analyzed than optical absorption spectral measurements.
Therefore, it is desirable to provide an apparatus and method that uses refractive index measurements to estimate one or more characteristics of brine obtained from formations that have water-based invasion, to determine purity or contamination of brine while withdrawing the fluid from a formation, and to estimate at a given time how long it might take before clean up will occur so that a sample may be taken. In certain situations, such as when the invaded zone is too deep or when there is continued penetration of an unwanted fluid from adjacent formations during the pumping of the fluid, it may not be feasible to obtain a clean sample even if pumping were to continue for a relatively long time period. In such cases it is desirable to determine in a relatively short time that it may not be feasible to withdraw samples in a reasonable amount of time from the particular location in the wellbore.